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1991 300E 4Matic 160k
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236 Posts
Discussion Starter #1
On my low list of priorities, but something I want to fix eventually. I start my car temp reads -36 degrees or something then whole display goes blank . Sensor no good? I can order for like 90$ but could it be something else? And where are the connections located? Cable in pic (new)looks very long.
 

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Banned
1993 400E
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7,361 Posts
Cable connection is under the fuse box.

Gauge going blank is the gauge, you could need both.

I would do the gauge 1st.
 

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Cruise Control
'87 300TD/'90 300D/'94 Quattro/'89 Vanagon TDI/'01 EV Weekender VR6
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52,233 Posts
Does anyone know if Radio Shack or some other online electronics supply store has a generic temp sensor that can be soldered into place at the end of the cable? Given that it's just a "dumb" sensor and a few feet of two wire cable, it seems kinda needlessly expensive and a hassle to swap out the whole thing, if the end can be replaced with minimal effort. Thoughts?
 

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86 300e Anke
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1,252 Posts
Is it an actual 'smart' device or merely a thermocouple, and the 'smarts' are mounted on the instrument cluster?

If it is a dumb thermocouple then it SHOULD be possible to do... if one can be found that matches the expected values...

... and on that subject,

When I first got Anke (well, even now on ocassion) hers would sometimes start around 'normal' then (usually) drop quickly to like -135 (which is all it showed)... then soon after 'blank out'. I think the 'smarts' is saying 'This cant be right... best shut down'...
Recently it has been far more stable, if not reliable... tho Im certain the display needs some 'touch-up' soldering as most times it doesnt bother to say 'F', as well as saying '-' when it means/should be '+'...

Random thoughts. :p
 

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1987 Mercedes Benz 300 TD Turbo, 2002 CLK 55 AMG
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344 Posts
The sensor is carbon base RTD with a 2 - 10,000 ohm resistance.

The sensors are made with many different aspect vectors for programming and applications requirements.

A basic 2 - 10K ohm is suitable for testing on mercedes inputs.

Mike
 

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Cruise Control
'87 300TD/'90 300D/'94 Quattro/'89 Vanagon TDI/'01 EV Weekender VR6
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52,233 Posts
I've been playing with a few sensors I have in my pile, including an ambient sensor from an Audi and Chrysler, and some random temp sensor from another Audi. I have a known good MB ambient sensor, so I've been trying to daisy chain resistors to get the other sensors to dial down to the approximate resistance level. The Audi unit might be able to be used, but the Chrysler unit has too much resistance. I was really hoping the American car sensors could be used, 'cause they're really cheap--like between $4 and $7
 

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86 300e Anke
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1,252 Posts
I've been playing with a few sensors I have in my pile, including an ambient sensor from an Audi and Chrysler, and some random temp sensor from another Audi. I have a known good MB ambient sensor, so I've been trying to daisy chain resistors to get the other sensors to dial down to the approximate resistance level. The Audi unit might be able to be used, but the Chrysler unit has too much resistance. I was really hoping the American car sensors could be used, 'cause they're really cheap--like between $4 and $7
HOW high?
Dont forget -- putting resistance in PARALLEL LOWERS overall resistance!

value = 1 / (1 / (R1 + R2))

might make it/them work! :thumbsup:

The sensor is carbon base RTD with a 2 - 10,000 ohm resistance.

The sensors are made with many different aspect vectors for programming and applications requirements.

A basic 2 - 10K ohm is suitable for testing on mercedes inputs.

Mike

Is it linear? Is the 5k 'midpoint' considered 0/32?
Is HIGH resistance 'low' temp or does it follow low ohm= low temp?
 

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The display has a 47uF 16v capacitor which goes bad with age. The component costs about a dollar from digi key or other component vendor. If Radio Shack still existed you likely could find one there. Get a soldering iron and some solder and carefully remove and replace the bad capacitor. There is a DIY online detailing this. The hardest part is removing the instrument cluster. It requires fabricating two wire "hooks" with ha does to slide on each side of the cluster and patiently pull and rock the cluster from the dash. It takes some time but will eventually come out.
 

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Outstanding Contributor
1989 560SEC, 1989 560SEL, 1995 E420
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4,446 Posts
The display has a 47uF 16v capacitor which goes bad with age. The component costs about a dollar from digi key or other component vendor. If Radio Shack still existed you likely could find one there. Get a soldering iron and some solder and carefully remove and replace the bad capacitor. There is a DIY online detailing this. The hardest part is removing the instrument cluster. It requires fabricating two wire "hooks" with ha does to slide on each side of the cluster and patiently pull and rock the cluster from the dash. It takes some time but will eventually come out.
Yes, this is an old post. However:

I replaced the cap in my temp gauge (1995 E420, was reading 18deg too cold). Didn't make a difference. Guess my issue is at the sensor. Anyone take the sensor apart yet?

By the way, I got the cap at Radio Shack. Still in business here in Norcal.
 

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Premium Member
1990 Benz 300E 2.6 (mine) ; 1994 E320 Wagon (wife's) ; 1993 300CE Cabrio, needs some TLC
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7,150 Posts
the sensor is just a thermister, nothing to see there. I'd read the resistance at the connector to the back of the display module... if its in range, your module is bad.

I picked up a new module for my 300E on fleabay, from a guy who said he tested them and showed a picture with it hooked to a test jig and actually reading out... he had several.
 
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